1. Field of the Invention
The present invention relates to a wire for arc welding, and in particular, to a wire for arc welding, which has enhanced feedability by uniformly distributing residual stress of the final wire product.
Wire acts as a filler metal for the mechanism of arc welding. To be specific, wire is wound around a spool or a pail pack for welding, and passes through a feeding roller and a welding torch cable in welding. The wire is then melted by an electric arc heating so as to be welded with a base metal. Therefore, it is important to provide a high feedability for stable welding to the wire. Further, in the light of the recent welding work seeking automation and high efficiency, it is mandatory to provide a stable feeding of wire in a rapid feeding velocity. Thus, the demand for enhancing feedability of wire is increasing.
2. Description of the Related Art
In a variety of wires including the one for arc welding, an initial rod passes through dies of diverse sizes, and is drawn to be a final wire product after undergoing steps of reducing diameter thereof to be thinner.
In the wire drawing process, the factors related to wire feedability may be a wire drawing schedule in accordance with the diameter reducing rate for drawing the wire having a desired diameter, distribution of internal stress through adjustment of deviation of the tensile strength and the elongation of wire, straightness of wire, etc. Of those, an uniform distribution of internal stress of the wire is a critical factor to be considered in enhancing the wire feedability.
The conventional method of controlling a wire drawing process to enhance feedability of the wire was limited to considering the diameter reducing rate only to reduce a diameter of the wire or an uniform distribution of the internal stress by adjusting the tensile strength or the elongation of the wire.
As the drawing of wire is repeated in the wire drawing process, however, the external portion of the wire, i.e., the outer surface of the wire, with which the dies are in contact, becomes denser than central portion of the wire and is hardened. As the surface of the wire is hardened, it is impossible to draw a wire, and the distribution of residual stress between the outer surface and the central portion of the wire becomes irregular. Therefore, the conventional control focused on a mere adjustment of the wire drawing schedule in accordance with the diameter reducing rate or an adjustment of the tensile strength has a limit in achieving a uniform distribution of residual stress between the outer surface and central portion of the final wire product.
Further, the hardness of the outer surface of the wire resulting from the repeated drawing thereof causes an abrasion of the dies, which are in contact with the wire, and causes irregular and damaged surface of the drawn wire, thereby lowering the quality of a final-wire product and interrupting a smooth feeding of the wire in the course of welding.
The abrasion of dies caused by contacting with the wire having a hardened outer surface results in an irregular contact area, which is in contact with the wire, and further results in an irregular distribution of the residual stress in the longitudinal direction of the final wire product. Accordingly, when the wire passes through a feeding roller and a welding torch cable in the course of welding, the load is partially concentrated, thereby causing failure of wire feeding because of entanglement and twist of the wire.
It is, therefore, an object of the present invention to enhance a feedability of a wire for arc welding by uniformly distributing an internal stress of the wire by adjusting the hardness difference in a radial and a longitudinal direction of the wire in the wire drawing process.
Another object of the present invention is to provide a wire for arc welding having a uniform distribution of residual stress of the wire by controlling an area of the wire contacting with dies, and by reducing the hardness difference of the wire.
Still another object of the invention is to provide a method of drawing a wire for arc welding that divides the final wire drawing step in an ordinary wire drawing process into two steps, whereby a hardness difference between a central portion and an outer surface of the wire is reduced through adjustment of the contact angle between the wire and the dies in the first step, and the hardness deviation in the longitudinal direction of the wire is reduced through adjustment of the length of a bearing part in which the wire is corrected.
To achieve the above objects, there is provided a wire for arc welding manufactured by firstly drawing a wire material, heat treating the drawn wire material for work hardening of it, secondary drawing the heat treated wire material, heat treating the secondary drawn wire material for removing internal residual stress of it, and finally drawing the heat treated wire material, wherein the final drawing step is carried successively out by a first die having a diameter reducing contact part of a smaller contact angle and a shorter bearing part for actually reducing a diameter of the wire to be worked, and a second die having a longer diameter correcting contact part than that of the first die for actually correcting the diameter of the wire, the second die being disposed in series with the first die, whereby a hardness difference between a central portion and an outer surface of the wire is less than 18 and a hardness difference between portions at intervals of 200 mm in the longitudinal direction is less than 15, when the hardness of respective wire portions is measured by Vickers Hardness tester.
The differences of hardness values are adjusted to the values when wire contact area rate defined the following formula is limited within a range of 3 to 3.5:
wire contact area rate=a diameter reducing contact rate+a diameter correcting contact rate 
diameter reducing contact rate=area of part for reducing a diameter of the wire/cross section area of the wire incoming into the first die
diameter correcting contact rate=area of the part for correcting a diameter of the wire/cross section area of the wire drawn from the second die.
As a technical concept to achieve the above objects, there is also provided a method of drawing a wire for arc welding to have a desired diameter, method for manufacturing a wire for arc welding, which comprises steps of firstly drawing a wire material, heat treating the drawn wire material for work hardening it, secondary drawing the heat treated wire material, heat treating the secondary drawn wire material for removing internal residual stress of it, and finally drawing the heat treated wire material, wherein the final drawing step includes steps of reducing a hardness difference in a radial direction of the wire using a first die having an area reduction contact part of a smaller contact angle and a shorter bearing part, and reducing a hardness difference in a longitudinal direction of the wire using a second die having a longer diameter correcting contact part than that of the first die and being disposed in series with the first die.